The catalytic site thiol-containing tryptic peptide of Escherichia coli UDP-D-glucose dehydrogenase (BL UDPGDH) will be sequenced by the Edman-dansyl technique. The relationship between incorporation of 14C-phenylglyoxal, loss of arginyl moieties and catalytic activity will be determined. 2,3-Butanedione-treated bovine liver UDP-D-glucose dehydrogenase, previously labeled at the catalytic site thiols will be subjected to tryptic digestion and the sequence of the resultant peptides which contain the catalytic site sulfhydryl and the modified arginyl moieties will be determined.